This invention relates to soldering fluxes and methods of use. The invention is more particularly concerned with a rosin-free, low VOC (volatile organic compound), no-clean soldering flux and with a method using the soldering flux to produce soldered printed wiring assemblies.
The present invention represents a further development from the invention described in commonly assigned U.S. Pat. No. 5,297,721 issued Mar. 29, 1994, incorporated herein by reference. Among other things, the earlier-patented invention provides a rosin-free, substantially VOC-free flux based on the use of a halide-free, water soluble activator in combination with a fluorinated surfactant and water. The preferred activators include carboxylic acids, such as glutaric acid and succinic acid, having solubilities in water of at least 5 grams per 100 cc at 20.degree. C. The flux is described principally for spray application to printed wiring assemblies preparatory to wave soldering, a commonly used soldering technique in which surfaces of a printed wiring (printed circuit board) and of component leads and terminations to be soldered thereto are subjected to a solder wave which passes along the board and components mounted thereon. The flux is applied to the assemblies in controlled amounts consistent with the attainment of an effective fluxing action with minimal post-soldering flux residue. This ensures minimal residual ionic contamination of the soldered assemblies so that post-soldering cleaning of the assemblies is unnecessary.
The earlier-patented flux has been found to provide excellent results under standard surface insulation resistance testing procedures for printed wiring assemblies, such as IPC and Bellcore procedures, as well as under standard corrosion resistance testing procedures, such as copper mirror and silver chromate paper tests.
Certain electronics manufacturers, however, utilize flux qualification test procedures which go beyond the generally accepted standards. These more rigorous procedures may involve, for example, the application of excessive amounts of flux to test boards, testing with no soldering, or testing with soldering but only minimal preheating of test boards. For the purposes of these testing procedures, we have found it desirable to provide a flux which can be formulated to achieve an even more benign residue than the preferred formulations of the earlier-patented flux.
The present invention has been devised in view of the foregoing.